Fen Management Handbook - Scottish Natural Heritage

Disadvantages of vegetation community mapping include:
– Skills in botanical surveying and accurate position fixing are needed for all
phases of survey and monitoring.
– The distinction between one type of vegetation and another similar community
can be subjective and problematic. For example, the transition from moist,
seasonally waterlogged soils to permanently wet soils and then to standing
water may be the zone of transition from swamp to tall herb fen communities in
the NVC classification. This could be a transition from S27 Potentillo-Caricetum
rostratae tall herb fen to S9 Caricetum rostratae swamp, both visually dominated
by bottle sedge. Similar transitions occur in other pairs of communities with a
common visually dominant species, such as common reed.
– The community definitions derive from a national sampling programme, and are
assumed to apply to the survey site; the monitoring programme can thus be hard
to defend from adversarial challenges as the site vegetation on the site is not
nationally identifiable. This applies particularly to the north of the UK..
– Significant change in the composition of a community can occur, such as the
loss of infrequent species, without appearing in the monitoring results.
– At least three, and preferably more, quadrats should be recorded for each
apparent community in all compartments or units of the fen, at the baseline
stage. For effective monitoring this then needs to be repeated for the key
communities, which is time consuming.
– Assumptions about the community type can bias surveying; typically, as fen
communities will be expected in fens, change to wet grasslands may be missed.
Accurate mapping of the distinct boundaries between communities is usually
beyond the limits of accuracy for hand held GPS units (normally +/- 3 m). This
introduces two sources of error, firstly in determining the actual boundary of a
community and secondly in determining its location. GPS errors from hand held
sets may be systemic and temporal; points along the same line may be recorded
with a consistent difference on different days. This problem is not restricted to
the GPS system however, since fens are not easy places to use any topographic
surveying method. Differential GPS systems, which are more accurate versions
of GPS using a fixed base station to provide a corrective signal, usually provide
the most accurate practical option. Hiring the necessary equipment may be more
practical than purchase because of the capital cost.
A recent development is the availability of a wide area corrective signal, from the
European EGNOS satellite system. Hand held sets that are WAAS enabled, and
able to receive EGNOS signals, are typically accurate to +/- 3 m. This may be
accurate enough for monitoring, depending on the extent of the community and the
overall size of the habitat. A 12 figure grid reference (1 m accuracy) for a known
point can enable the GPS reading to be checked at the start and finish of work.
A simpler method may be to use anchored fixed points such as dipwells (see below)
or deeply driven stakes, with measured distance and direction to the apparent
community boundary.
Fixed point photography is very useful for monitoring major vegetation changes such
as scrub or typha encroachment. Locations for fixed point photography need to be:
– Representative of the fen area to be monitored;
– Relatively easily accessible without risk of damage getting to the location;
– Marked with a fixed post or reference point in the resulting picture e.g. a certain
tree which will always be photographed in the top right hand corner, and a small
marker post in the bottom left of the picture.
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